Upflow continuous backwash filters (UCBF's) employ a sand bed through which wastewater ascends. During this process, suspended solids are removed and the treated wastewater collects in a filtrate trough and flows out as treated effluent. Sand, the filter medium, which has caught suspended solids is drawn from the bottom of the UCBF through an air lift pipe and cleaned while ascending together with air and water. The sand is separated from the wastewater at a separation section and then further cleaned with filtered water flowing upwards while the sand falls down through a cleaning section. Thereafter, the sand is uniformly scattered across the top surface of the sand bed by a sand distributor creating a fresh layer of sand capable of filtering suspended solids.
UCBF's provide a stable treatment process for providing filtered water. The wastewater treatment process operates on a continuous cleaning principle, producing stable and good-quality filtered water under a constant pressure drop. UCBF's have the capability to filter highly contaminated wastewater. Due to their simple structure requiring little supplementary equipment, maintenance of UCBF's is straightforward and operating costs are comparatively low.
Nevertheless, in view of ever more stringent limits on pollutants that may be contained in purified drinking water, a demand exists for UCBF's that can remove larger amounts of pollutants from wastewater, in particular to remove nitrogen present as nitrate or nitrite. However, it is often necessary to add consumables, such as a carbon source, to the wastewater to increase the nitrogen removal efficiency of the carbonaceous bacteria. Hence, it is desirable to reduce the amounts of consumables necessary for removal of pollutants, especially in view of a potential price increase of these consumables over the operating life of a UCBF.
The upflow continuous backwash filters known from the related art, however, have certain disadvantages. For example, the typical operation of a related art UCBF creates inefficiencies in the nitrate nitrogen removal process because the sand media cleaning process that occurs in the continuous backwash upflow column pipe not only scrubs the sand media but also removes the carbonaceous bacteria that grow on the biomass of the sand media from the wastewater treatment process. FIG. 1 of U.S. Pat. No. 7,381,336 provides for a system in which reject from washer 82 is discharged through an outlet port 46. The reject is therefore removed from the system depicted in FIG. 1 of U.S. Pat. No. 7,381,336. Moreover, U.S. Pat. No. 7,381,336 provides for a method in which mechanical agitation and turbulence caused by the action of the air bubbles in air-lift pump 76 are so intense that microorganisms are killed. Such a system is, therefore, not suitable for recycling of microorganisms.